In prior-art laser illuminated displays, a common arrangement for reducing speckle contrast includes providing diversity of incidence angle, polarization or wavelength of the beam, alone or in combination, at each point of the display screen, in order to form a number N of different and uncorrelated speckle patterns. In an arrangement described in U.S. patent application Ser. No. 11/011,736, file Dec. 14, 2004, angular diversity of the beam is provided and a condition of un-correlation is achieved by separating, in time, the beam incident on the screen at different angles, with the range of different angles being presented within the normal integration time period of a human eye. This arrangement is preferred for displays including a two-dimensional spatial light modulator, although the arrangement is not ineffective in a display including a one-dimensional light modulator. A one-dimensional light modulator is different from a two-dimensional light modulator in terms of requirements for a light source illuminating the modulator and for speckle-reduction means.
One preferred prior-art arrangement for speckle reduction in a display including a one-dimensional modulator includes the use of a vibrating phase mask. A brief description of such a prior-art display is set forth below with reference to FIG. 1.
Here a display 20 is illuminated by a red laser, a green laser, and a blue laser (not explicitly shown). Beam-forming optics 22, 24, and 26 form a line-shaped illumination pattern from respectively the red, green and blue laser light on one-dimensional (linear) spatial light modulators 28, 30, and 32, respectively. The length dimension of the modulators is perpendicular to the plane of FIG. 1
In this example, it is assumed that the linear modulators are diffractive modulators of the grating light-valve (GLV) type. This type of modulator includes a plurality of microscopic reflective elements or beams arranged parallel to each other in a linear array. The reflective elements can be individually raised and lowered by electrostatic attraction. The reflective elements function as a diffraction grating that can cause varying amplitudes of diffraction along the line of light incident thereon. The varying diffraction amplitudes represent image information in one line of a display image to be projected. Light reflected and diffracted from the three modulators is combined by beam combining optics 34, for example, a Philips prism, and directed to imaging optics 36.
Imaging optics 36 include lenses 38 and 40 having a mask 42 therebetween that passes light diffracted into the +1 and −1 orders of the diffraction grating, and rejects light diffracted in the zero order (specularly reflected) of the diffraction grating. Mask 42 is usually referred to as a Schlieren filter or a Fourier filter by practitioners of the art. Imaging optics 36 create an intermediate composite image of the three modulators, using the +1-order diffracted light therefrom, on a speckle-reduction arrangement 44. Speckle-reduction arrangement 44, in this example, is assumed to be a reciprocating phase-mask. The intermediate image is re-imaged by projection optics 46 via a galvanometer scanning mirror 48 on a screen 50. The projected image forms one line of a display to be projected on the screen, the line, here, having a length perpendicular to the plane of FIG. 1. Galvanometer mirror 48 is scanned, as indicated by arrows A, step-wise, projecting a new line of the image at each step. A complete scan between positions 52A and 52B is performed sufficiently fast that the sequentially projected line-images appear to a viewer as a two-dimensional image. The two-dimensional image is a bit map having as many width elements as there are beams (grating lines) in each the linear modulators, and as many height elements as there are sequentially projected images on the screen. A one-dimensional GLV modulator can have as many as 1080 beams.
In a phase-mask speckle-reduction arrangement such as arrangement 44, a plate having a spatially varied thickness providing a particular ordered phase pattern is rapidly, reciprocally translated at the intermediate image position. The rate of reciprocation is selected such that this causes uncorrelated phase patterns representing image points on the screen to be averaged at a rate that exceeds the resolution limit of the eye. This requires mechanically translating the phase mask in the beam, so that a maximum number N (for example, N=64) of uncorrelated phase patterns can be presented within the integration time. The maximum number of patterns that needs to be presented is dependent on the maximum possible ratio of speckle contrast reduction rmax. The value of rmax, in turn, is determined by the ratio of a solid angle Ωtot subtended by the projection optics at the screen, to a solid angle Ωeye subtended by the observer's pupil at the screen. The following relationships exist for N, rmax, Ωeye, and Ωtot.rmax=(Ωtot/Ωeye)1/2  (1)N<rmax2  (2)
The projection optics speed (Ωtot) is limited, by the acceptance angle of the one-dimensional modulators, and the size and cost of the projection lens, among other factors. Only uncorrelated patterns are effective in determining the relationships. Accordingly, presenting more correlated patterns will not increase speckle contrast reduction. The object of the reciprocating phase-mask is to be able to present N˜rmax2 phase patterns that lead to N uncorrelated speckle patterns at the screen, within the integration time of the human eye. This results in an “optimal” speckle reduction, meaning a reduction equal to rmax, with a minimal number of patterns N, which translates to a minimal time required to present these patterns. The minimal time is important, and this is what makes the case of one-dimensional modulator much more challenging for speckle reduction, compared to a two-dimensional modulator. The reason is that the laser beam actually illuminates each particular resolvable spot of the screen only for a fraction of the total integration time of the eye, due to the scanning. Accordingly, a “non-optimal” set of patterns, for example, a set that includes mutually correlated patterns, may require more time to present, and lead to a lesser reduction ratio.
One disadvantage of this speckle reduction approach is that the phase-mask must be reciprocally translated at high frequency, for example about 30 Hz or greater, in the intermediate image plane. This requires a complex, delicate mechanism. A related disadvantage is that imaging optics 36 has to form the image in the intermediate image plane, which adds to the complexity and cost of optics in the display. An additional difficulty with one-dimensional modulators, in general, is that the line of light projected from a laser onto a modulator must be well focused in one plane, as a modulator is typically only about 25 micrometers (μm) wide. If a single-mode laser is used as an illuminating laser this does not present a problem, however, it is usually preferred to a multimode diode-laser arrays (bar), including a plurality of individual emitters, as the illuminating laser. One reason for this is the simplicity, cost, and available power of such a bar. In a diode-laser array, the “fast” axis of the output beam (perpendicular to the length of the array) is readily focusable, but individual emitters in the array are often misaligned in the length direction by several micrometers, an effect whimsically referred to as the “smile” of the array by practitioners of the art. This can create difficulty in imaging the length of the array on the modulator, which is usually exactly straight.
There is a need for a one-dimensional modulator display system, including a speckle-reduction arrangement that does not include any moving parts. The speckle-reduction arrangement should be suitable for illuminating a one-dimensional modulator using multimode lasers in general, and using diode-laser arrays in particular.